Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). In some examples, a portion of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE control system may disable selected cylinders through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling.
Reducing the number of active cylinders may also reduce the operating temperature of various engine and/or vehicle components, potentially degrading engine operation. For example, when transitioning from VDE mode (or partial cylinder mode where one or more cylinders are deactivated) to a non-VDE mode (or full cylinder mode where all the cylinders are active), individual cylinder load (based on aircharge) may decrease. This may cause certain emission control devices, such as catalytic converters, to cool below a minimum operating temperature required for efficient operation.
One example approach to effectively manage the temperature of catalytic converters and other emission control devices in a VDE engine is shown by Glugla et al. in U.S. Pat. No. 6,415,601. Therein, one or more deactivated cylinders on a deactivated engine bank are reactivated in response to an emission control device temperature falling below a threshold. By reactivating the cylinders, a temperature of an exhaust catalyst is rapidly raised.
However the inventors herein have recognized a potential issue with such an approach. Preponing reactivation of cylinders to raise an emission control device temperature may result in decreased fuel economy. Specifically, by not allowing the engine to operate in the partial cylinder mode for a longer duration, the fuel economy benefits of the VDE engine are not optimized.
In one example, the above issue may be at least partly addressed by a method for an engine comprising: selectively deactivating one or more engine cylinders responsive to operating conditions, and during the deactivation, monitoring a temperature of an emission control device coupled downstream of the one or more engine cylinders. In response to the temperature falling below a threshold, the method includes operating active cylinders with split fuel injection. In this way, VDE operation can be prolonged without degrading exhaust emissions.
In one example, a variable displacement engine may be configured with selectively deactivatable fuel injectors. In response to selected deactivation conditions, such as reduced engine load or torque demand, one or more cylinders may be deactivated and the engine may be operated in a VDE mode. For example, the engine may be operated with half the cylinders deactivated. During the deactivation, a temperature of an emission control device (e.g., an exhaust catalyst) coupled downstream of the engine cylinders may be monitored, such as via an exhaust temperature sensor. As such, during the deactivation, the temperature of the emission control device may fall. If the temperature falls below a threshold, such as a light-off temperature, additional exhaust heat may need to be generated to reactivate the emission control device and reduce exhaust emissions. Accordingly, in response to the drop in temperature, the controller may transiently shift fuel injection of the active cylinders from a single fuel injection to a split fuel injection. In addition to using split fuel injection, ignition timing may be retarded and/or the engine may be operated with exhaust valve opening retarded to maximize heat flow to the exhaust catalyst. For example, for a number of combustion events since the drop in catalyst temperature, the active cylinders may be operated with fuel delivered as at least a first intake stroke injection and second compression stroke injection while spark timing is retarded. A split ratio of fuel delivered in the first intake stroke injection relative to fuel delivered in the second compression stroke injection may be adjusted based at least on the temperature of the exhaust catalyst (e.g., a difference between the temperature of the exhaust catalyst and the threshold temperature). By temporarily shifting to a split fuel injection, reactivation of the exhaust catalyst can be expedited, improving exhaust emissions.
In some embodiments, such as where the engine was operating with EGR during the VDE mode of operation, the split injection may also be adjusted based on the EGR. For example, split injection may be continued while the EGR is bled down.
It will be appreciated that during some conditions, such as where the fuel penalty associated with the split fuel injection is higher than a threshold, the controller may reactivate all the engine cylinders instead of shifting the active cylinders to the split fuel injection. Therein, the reactivation of all the engine cylinders may be a more fuel economical way to raise exhaust temperature.
In this way, by operating active cylinders with split fuel injection for a number of combustion events during a VDE mode of engine operation, the temperature and catalytic efficiency of an exhaust catalyst can be rapidly recovered. By using the split injection to expediate exhaust warming, the need for reactivating all engine cylinders is reduced. As such, this prolongs a duration of engine operation in the VDE mode and enables fuel economy benefits to be achieved over a longer period of vehicle operation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.